Earth's northern and southern hemispheres, when viewed from space, appear equally bright. This is unexpected and mysterious because the Southern Hemisphere is mostly covered with dark oceans, while the Northern Hemisphere has a vast land area that is much brighter than these oceans. The brightness symmetry between the hemispheres has remained a mystery for years. A new study has revealed a strong correlation between storm intensity, cloudiness and the solar energy reflection rate in each hemisphere. Not only has the study offered a solution to the mystery of the hemispheres appearing equally bright from space, but also provided an assessment of how climate change might alter the reflection rate of solar energy in the future.
The study, led by Weizmann Institute of Science, was recently published in the Proceedings of the National Academy of Sciences (PNAS).
What is albedo?
When scientists analysed data from the first meteorological satellites in the 1970s, they were surprised to find out that the two hemispheres reflect the same amount of solar radiation. In scientific terms, the reflectivity of solar radiation is called "albedo".
One can understand albedo through the example of driving at night. One can spot intermittent white lines easily, because they reflect light from the car's headlights well, but find it difficult to discern the dark asphalt.
Why the Northern Hemisphere and Southern Hemisphere appear equally bright from space
In the case of observing Earth from space, the ratio of the solar energy hitting the Earth to the energy reflected by each region is determined by various factors. One of the factors is the ratio of dark oceans to bright land. The dark oceans and bright land differ in reflectivity, just like asphalt and intermittent white lines. The land area of the Northern Hemisphere is about twice as large as that of the Southern Hemisphere. When measuring reflectivity near the surface of the Earth, when the skies are clear, there is more than a 10 per cent difference in albedo. Yet, both the hemispheres appear to be equally bright when viewed from space.
Another factor influencing hemispheric albedo
As part of the new study, the researchers focused on another factor influencing albedo: clouds. Clouds are located in high altitudes and reflect solar radiation. The researchers analysed data derived from the world's most advanced databases, including cloud data collected using NASA satellites launched under the Clouds and Earth's Radiant Energy System (CERES) project, and data from ERA5, which is a global weather database containing information collected using a variety of sources in the air and on the ground, dating back to 1950. The researchers used ERA5 data to complete cloud data and to cross-correlate 50 years of the data with information on the intensity of cyclones and anticyclones.
Based on intensity, the scientists classified storms of the last 50 years into three categories. The scientists discovered a direct link between storm intensity and the number of clouds forming around the storm. The Northern Hemisphere and land areas, in general, are characterised by weaker storms. Meanwhile, above the oceans in the Southern Hemisphere, moderate and strong storms prevail. Since there is a link between storm intensity and cloudiness, there is a difference in cloudiness between the two hemispheres, data analysis showed.
In a statement released by Weizmann Institute of Science, Or Hadas, one of the authors on the paper, said cloud albedo arising from strong storms above the Southern Hemisphere was found to be a high-precision offsetting or counteracting agent to the large land area in the Northern Hemisphere. Hence, the symmetry is preserved.
Hadas said this suggests that storms are the linking factor between the brightness of Earth's surface and that of clouds, solving the symmetry mystery.
Can climate change make one of the hemispheres appear darker?
In recent areas, Earth has been undergoing rapid changes due to climate change. The environment has been severely affected, and climate change-induced extreme weather events have increased.
In order to examine whether and how climate change could affect hemispheric albedo symmetry, the scientists used CMIP6. This is a set of models run by climate modelling centres around the world to simulate climate change. One of the major shortcomings of these models is their limited ability to predict the degree of cloudiness.
The researchers could assess future cloud amounts, based on storm predictions, with the help of the relation found in the study between storm intensity and cloudiness.
Global warming will result in a decreased frequency of all storms above the Northern Hemisphere and of weak and moderate storms above the Southern Hemisphere, according to predictions by models. However, there will be an intensification in the strongest storms of the Southern Hemisphere.
Arctic amplification is the cause of these predicted differences. Arctic amplification is a phenomenon in which the North Pole warns twice as fast as Earth's mean warming rate.
One might think that the difference in the frequency of storms between the two hemispheres will break hemispheric albedo symmetry.
However, since cloud amounts reach saturation in very strong storms, a further increase in storm intensity might not change the degree of cloudiness in the Southern Hemisphere. Therefore, hemispheric albedo symmetry might be preserved.
Professor Yohai Kaspi, who led the research, said it is not yet possible to determine with certainty whether the symmetry will break in the face of global warming. However, he said, the research solves a basic scientific question and deepens researchers' understanding of Earth's radiation balance and its effectors.
Kaspi said he hopes that a better understanding of basic climate phenomena, such as the hemispheric albedo symmetry, will help in developing geoengineered solutions to help human life to carry on alongside global warming.